1. Field of the Invention
The present invention relates to the coupling of signals across capacitors.
2. Background Art
It is sometimes necessary to couple a signal between two or more electrical circuits that operate at different voltages. Circuitry for coupling a signal under such circumstances must provide for the isolation of the voltages of the different circuits to prevent interference or damage to the circuits from voltages exceeding the intended operational voltage ranges of the circuits. For example, when interfering a computer to a telephone line, such as with a modem, it is necessary to provide isolation between the circuits of the computer, which operate at controlled low voltages, and the telephone line, where voltages over a very wide range, including high voltages, may be present. While such isolation is necessary, it must be accomplished in a manner that allows the coupling of signals between the computer and the telephone line without undesirable levels of distortion, attenuation, or other degradation of the signals.
Traditionally, such isolation was provided using an isolation transformer. An isolation transformer magnetically couples signals between its windings while keeping the windings electrically isolated from one another. Since DC components of a signal do not pass through the isolation transformer, DC potentials on opposite sides of the isolation transformer are isolated from one another.
An isolation transformer comprises a core of magnetic material and a plurality of windings. Since the minimum size of the core and windings is dictated by the desired electrical parameters of the transformer, the size of the transformer cannot be reduced below that minimum. Thus, isolation transformers are typically relatively large and bulky. They often have dimensions that are inconsistent with miniaturized packaging configurations. Isolation transformers are also usually rather expensive since the require production and assembly of specialized magnetic materials, insulated wire, insulating materials, and connection terminals.
FIG. 1 is a diagram of a telephone line interface circuit of the prior art. Telephone line 105, which comprises tip and ring conductors, is coupled to protection devices and DC holding circuit 101. Protection devices and DC holding circuit 101 is coupled to node 108, which is coupled to a first terminal of a first winding of transformer 103. Protection devices and DC holding circuit 101 is also coupled to node 109, which is coupled to a second terminal of the first winding of transformer 103. A first terminal of the second winding of transformer 103 is coupled to node 110, which is coupled to transmit and receive circuits 102. A second terminal of the second winding of transformer 103 is coupled to node 111, which is coupled to transmit and receive circuits 102. Transmit signal TX is applied to node 106, which is coupled to transmit and receive circuits 102. Transmit signal TX is applied to node 106, which is coupled to transmit and receive circuits 102. Transmit and receive circuits 102 are coupled to node 107, which provides receive signal RX.
The first winding and the second winding of transformer 103 are isolated from each other so as to maintain high voltage barrier 104. Thus, great voltage differences between the circuits on the two sides of high voltage barrier 104 may exist without harm or interference to circuits on the other side of high voltage barrier 104.
Any signals desired to be transmitted over telephone line 105 are applied as signal TX at node 106. The signal to be transmitted is processed by transmit and receive circuits 102 and applied across nodes 110 and 111. The signals are coupled across coupling transformer 103 to nodes 108 and 109. The signals pass through protection devices and DC holding circuit 101 to be applied to telephone line 105. Signals on telephone line 105 pass through protection devices and DC holding circuit 101, through nodes 108 and 109 to transformer 103. Transformer 103 couples the receive signals to nodes 110 and 111, which couple the receive signals to transmit and receive circuits 102. Transmit and receive circuits 102 output the receive signals at 107, which provides receive signal RX.
Another approach involves coupling signals with optocouplers. Optocouplers (or optoisolators) include an LED having its light output coupled to a phototransistor, photodiode, or other photosensitive device, with the assembly encapsulated in an opaque package. Since optocouplers rely on LEDs, which do not exhibit linear response, optocouplers are generally used to transmit signals of a digital nature, which do not require linear devices for transmission. Moreover, reasonably priced optocouplers are generally too slow to provide sufficient transmission quality.
Capacitors are used to couple signals. Since the frequency range of the signals that may be coupled is related to the value of the capacitors, only signals of certain frequency ranges may be coupled by capacitors of particular values. Capacitors typically have a size proportional to their value. Also, there is typically an inverse relationship between the value of a capacitor and the frequencies that it will pass. Thus, a large capacitor is usually required to pass low frequency signals, for example, audio frequencies. Large capacitors are often too large and bulky to be accommodated by miniaturized packaging configurations. Large capacitors are also more expensive.